The present invention relates generally to improvements in an illumination control apparatus in an optical projection device such as a microreader, reader-printer, or the like, and it relates more particularly to an improved illumination control apparatus for providing a constant illumination and a uniform distribution of illumination in the plane of the image formed by an optical projection device independent of variations in the projection magnification of the device.
There are various standards of reduction ratio for microfilms which are widely used for recording literature, papers, documents, etc. and films of many different sizes and reduction ratios are in use. When such microfilms are enlarged and projected with a single reader or reader-printer for viewing or copying, a lens is selectively used which has a projection magnification corresponding to the size or reduction ratio of the particular film to be projected. Thus, whenever the projection lens is changed, the area of the projected image on the screen or at the exposure station involves a variation in the illumination and in the distribution of illumination due to the difference in the projection magnification and the f-number of the lens.
With conventional reader-printers, therefore, the power supply to the light source is altered with a change of the projection lens to maintain the illumination or the amount of exposure at a constant level on the screen or at the exposure station, as described in U.S. Pat. No. 4,172,657.
In the case of such a reader-printer, a plurality of projection lenses are mounted on a changeover plate at predetermined locations, so that each time a projection lens is shifted or another projection lens is used, the power supply must be manually adjusted in corresponding relationship with the selected lens. Accordingly, the use of many different projection lenses requires the frequent manual adjustment of the power supply which is highly inconvenient and cumbersome.
Futhermore, when the power supply energizing the light source is merely controlled, it is impossible to provide a constant or uniform distribution of illumination to the image projected onto the screen or exposure station, and an uneven exposure will occur at the exposure station.
U.S. Pat. Nos. 3,472,586 and 4,029,956 issued Oct. 14, 1969 and June 14, 1977, respectively, describe a device for moving the light source to alter the distance between the condenser lens and the light source and a device for altering the distance between the condenser lens and the light source in accordance with variations in the projection magnification. However, difficulty is encountered with such devices in maintaining a constant or uniform illumination on the screen or at the exposure station regardless of the projection magnification.